1. Field of the Invention
The present invention relates to a chroma phase modulator for use in a time base corrector in video equipment such as video tape recorders and laser disk players.
2. Description of the Related Art
FIG. 8 illustrates a conventional chroma phase modulator, and FIG. 9 illustrates the operating principle of the circuit of FIG. 8.
In FIG. 8, a laser disk playback section 10 plays a video signal and an audio signal back from a laser disk LD. A composite video signal Vs containing a luminance signal and a chroma signal (e.g., Y + A sinX, Y: luminance signal, A sinX: chroma signal), which is output from the laser disk playback section 10, is applied to an analog-to-digital (A/D) converter 11. The A/D converter 11 is supplied with a clock signal CK having a frequency of 4 fsc (fsc: the color subcarrier frequency of, say, 3.58 MHz) from a phase locked loop (PLL) circuit 12, converting the composite video signal to a digital signal in accordance with the clock signal CK. The composite video signal in digital form is applied to a bandpass filter (BPF) 13 that extracts the chroma signal (A sinX) and a color burst signal from the composite video signal. The chroma signal is delayed in phase by 90.degree. by a delay circuit 14 which introduces a time delay of 1/4 fsc, resulting in -A cosX.
On the other hand, control data from a voltage controlled oscillator (VCO) included in the PLL circuit 12, i.e., the amount .theta. of phase modulation, is applied to a tan.theta. generator 15 for generating values of -tan.theta.. The phase modulation amount .theta. is generated by sampling the burst signal output from the bandpass filter 13. The tan.theta. generator 15 may comprise a read only memory (ROM) adapted to store values of tan.theta.. The values of -tan.theta. stored in the tan.theta. generator 15 are accessed by the phase modulation amount .theta.. The value of -tan.theta. read from the generator 15 and -A cosX output from the delay circuit 14 are applied to a multiplier 16 where they are multiplied. Assuming .theta.= tan.sup.-1 (B/A) and since tan.theta. = B/A, the product, B cosX, is output from the multiplier 16. The product B cosX and the chroma signal A sinX are applied to an adder 17. The output of the adder 17 is given by EQU A sinX+ B cosX=(A.sup.2 +B.sup.2).sup.1/2 sin(X+tan.sup.-1 (B/A))(1)
This output signal is tan.sup.-1 (B/A) out of phase with the chroma signal A sinX. Since tan.sup.-1 (B/A), the signal represented by equation (1) can be considered to be equivalent to phase modulation of the chroma signal by .theta..
It will be appreciated from the operating principle shown in FIG. 9 and equation (1) that the amplitude of the chroma signal phase modulated by the prior art circuit becomes (A.sup.2 +B.sup.2).sup.1/2, not A, that is, the phase-modulated chroma signal becomes larger in amplitude than the original chroma signal. If, therefore, the phase modulation amount is particularly great, great errors will be produced.